The present invention relates to a new and improved method for the catalytic manufacture of polyurethane prepolymers.
During the industrial fabrication of polyurethane prepolymers from polyisocyanates and compounds with active H-atoms, such as for instance polyols, controlling the reaction speed is problematic.
According to a known method the reaction speed is shortened in that, for instance, a diisocyanate-polyol mixture of the desired composition is heated between 110.degree. to 120.degree. C. and maintained for such length of time at this temperature until the polyurethane prepolymer possesses the desired NCO-value or until there have been consumed all of the isocyanate groups.
As a general rule the shortening of the reaction time which is obtained according to this process does not reach the economically desired degree. The high temperatures required for a technologically interesting shortening of the reaction time bring about undesired side reactions which result in that the desired end values of the repolymers, such as for instance viscosity, storage stability or NCO-value in the case of prepolymers, for instance with terminal NCO-groups, fluctuate in an uncontrolled manner over a wide range.
According to another known process catalysts or mixtures of catalysts can be employed for the shortening of the reaction time for the reaction of polyisocyanates with compounds having active H-atoms.
Known catalysts are, for instance, organometallic compounds, metal halides, acids, bases or tertiary amines. The publication of, for instance, J. H. Saunders and K. C. Frisch in "Polyurethanes", Chemistry and Technology, Part I, pages 129-217, Interscience, New York, 1965, provides a good insight into known catalysts.
The use of catalysts during the manufacture of polyurethane prepolymers has the advantage, in contrast to the first mentioned technique, that the reaction for instance of a diisocyanate with for instance a polyol can occur with only slightly elevated temperatures, so that for the most part undesired side-reactions brought about by high reaction temperatures are suppressed.
The use of catalysts during the production of polyurethane prepolymers is however problematic inasmuch as following interruption of the reaction process they further remain active and, for instance, in the case of NCO-terminated prepolymers cause intermolecular reactions. Consequently, the storage stability or self-life of such type prepolymers can be reduced that they are not capable of being stored or can no longer be processed even after a very short period of time. A further drawback of this method resides in the fact that the active catalysts contained in the prepolymer also can be active in the products which are produced with such prepolymer without it being possible to control such activity to a desired degree. The problem of inactivating catalysts has not yet been solved. It has indeed been proposed, for instance, to neutralize basic catalysts with acid chlorides, such as p-nitrobenzyl chloride (W. Cooper, R. W. Pearson, S. Drake, Ind. Chemist 36, 121 (1960)). The reaction products of such neutralization or the acids released during neutralization however can in turn again function as catalysts, so that prepolymers treated in this manner in toto do not exhibit the strived for improvements, for instance storage stability.